10.4.4 Energy Consumption
In battery powered systems, the energy consumed to complete a task is often a more relevant metric than power. Circuit designers often have the ability to trade-off power for performance. Thus it is possible for a high power system, which rapidly completes a task, to consume less energy than a low power system that steadily draws power for an extended period to complete the same task. Battery life for mobile systems can be extended by being energy efficient, not necessarily by being low power. Figures 10.2 and 10.3 showed that the perception processor has low power consumption and high performance. This in turn translates to a high degree of energy efficiency. Figure 10.4 shows the per packet energy consumption of the perception processor and its competition. While delivering 11.8 times the performance of the XScale processor, the perception processor consumes 13.5 times less energy than the XScale for each input packet. General purpose processors exact a high energy cost for their generality and programmability when compared to ASICs. From the results in Figure 10.4 it is possible to compute that on average the XScale consumes 79.3 times more energy per input packet when compared to the ASIC implementations (Gau, Rowley, FIR, Rijndael). In sharp contrast, the perception processor's energy consumption is only five times larger than that of the ASIC. The perception processor thus radically improves energy efficiency while retaining a high level of generality and programmability.